Fiction was not always given subject headings when cataloged. In the 1960s, children's fiction began to have headings so children could find fiction books as well as nonfiction in their searches. Adults soon wanted this feature to be added to their books as well.
"If a young patron wants to learn something about spiders, Charlotte's Web might
be a good choice, especially if reading a difficult nonfiction book without
pictures is the sole alternative. More mature readers can learn something about
British colonial rule in India by reading Kipling; Dickens reveals much about
social conditions in ninteteenth-century England, even though they are works of
Standard Cataloging for School and Public Libraries, Intner & Weihs, 4th edition.